1. Field of the Invention
The invention pertains to the field of forming metallic parts by compaction of powder metal. More particularly, the invention pertains to a novel apparatus and process to create substantially circular or eccentric undercuts that are perpendicular to the axis of the tooling ejection movements.
2. Description of Related Art
Powder metal compaction processing has led to the ability to manufacture workpieces having a variety of shapes and configurations without having to further machine certain features or dimensional characteristics into these workpieces. Powder metal compaction has become a popular means for producing gears having circumferential or even helical rows of teeth. One essential factor that must be taken into consideration, when designing the dies used to make such components or in selecting what type of component is to be made by this process, is that after the compaction process has formed the part, the dies must be capable of separating and freely ejecting the part.
Conventional powder metal compaction generally consists of a compaction press that houses a tool set. A typical tool set consists of a single die containing a cavity in the shape of the desired end product, one or more bottom punches to facilitate the formation of features on the bottom side of the product, one or more top punches to facilitate the formation of features on the top side of the product, and a core rod to facilitate the formation of one or more series of shaped inner diameters. Variations of this typical tool set may be employed to obtain variations in product shape.
One such tool set variation to compact a part out of powder material uses an upper die and a lower die. Each die houses at least one moveable punch that is capable of moving axially in response to pressure exerted in the axial direction by a compaction press. With this method, the bottom die and top die are initially positionable in contact with a lower punch engaged in the lower die to create a cavity for receiving powder material when the dies are in the closed position and the top punch is raised and separated from the top die. A powder feedshoe carries powder across the top surface of the top die and powder fills the cavity then created by the top die, the bottom die and the bottom punch. The feedshoe is retracted and the top punch is then introduced into the top die to start the compaction process.
One problem that has traditionally limited the broader use of powder metal compaction manufacturing is that the process generally produces workpieces that consist of features sculpted by the combination of the movement of the dies, the movement of the punches, and the process of removing the finished product wherein such movements are only performed in the axial direction. Workpieces having desired non-axial features, such as undercuts, are nearly impossible to produce without having to add secondary processing, such as machining or grinding, after the compaction process to remove material to create such features. Such examples of workpieces having non-axial features are circular shaped gears having at least two rows of circumferential teeth that are separated by a circumferential undercut. It is highly desirable to be able to produce such a part by a single process, such as a powder compaction process alone, rather than having to perform supplemental machining or grinding operations on the compacted part.
Attempts have been made to provide undercuts in powder metal compacted parts. One such process is disclosed in U.S. Pat. No. 4,087,221 in which a powder metal die is used to produce a part having undercut portions that are formed by using removable inserts. This process still requires additional operating steps since the inserts must be manually removed from the finished part after the completion of the compaction process. A variation on this concept is disclosed in U.S. Pat. No. 4,255,103. In this patent, annular flanges are formed in circular parts by the use of shaping inserts. However, once the part is removed from the compaction die, the shaping inserts must be removed by additional processing such as leaching or machining.
A recent approach to resolving the problem of providing undercuts in workpieces such as gears having dual rows of teeth is shown in U.S. Pat. No. 5,378,416. Disclosed therein is a die set consisting of a lower die, an upper die and a “cam die” that moves laterally across the top surface of the lower die. As the compaction process begins, the upper die punch and the lower die punch move axially toward each other to compress the powder metal in the cavity while the two opposing segments of the cam die move laterally toward each other to form a circumferential undercut between two rows of teeth. The problem with this design is that since the two cam die segments move toward each other along a single axis, differences are created in the density of the compacted powder metal part between the portions of the part adjacent the centers of each hemispherically shaped cam and the portions of the part adjacent the points of contact between the two cam dies. The density variance contributes to the uneven distribution of stresses on the part which can lead to premature fracturing and a shorter life cycle. In addition, upon the retraction of the two cams away from the die cavity after compaction of the part, the different portions of the cams move differently against the finished part. The centers of each cam slide radially away from the finished part while the ends of each cam slides away in a substantially tangential direction. These different sliding movements create different stresses on the workpiece with which each die portion is in contact until fully disengaged from the part. This difference may create the potential for the formation of unpredictable patterns of stress fractures. Further, if the laterally moving cam dies fail to meet completely, a gap is created which results in the formation of a “tab” or seam of excess material that must be removed by such means as machining.